Editing Manual transmission (section)

=== Reverse gear ===
Even in modern transmissions where all of the forward gears are in a constant-mesh configuration, often the reverse gear uses the older ''sliding-mesh'' ("crash box") configuration. This means that moving the gearshift lever into reverse results in gears moving to mesh together. Another unique aspect of the reverse gear is that it consists of two gears—an [[idler gear]] on the countershaft and another gear on the output shaft—and both of these are directly fixed to the shaft (i.e. they are always rotating at the same speed as the shaft). These gears are usually [[spur gear]]s with straight-cut teeth which—unlike the helical teeth used for forward gear—results in a whining sound as the vehicle moves in reverse.

When reverse gear is selected, the idler gear is physically moved to mesh with the corresponding gears on the input and output shafts. To avoid grinding as the gears begin to the mesh, they need to be stationary. Since the input shaft is often still spinning due to momentum (even after the car has stopped), a mechanism is needed to stop the input shaft, such as using the synchronizer rings for fifth gear. However, some vehicles do employ a synchromesh system for the reverse gear, thus preventing possible crunching if reverse gear is selected while the input shaft is still spinning.<ref>{{cite web|url=http://www.alfisti.net/311.2.html?&L=1 |title=Buyers Guide Alfa Romeo Spider & GTV 916 |publisher=Alfisti.net |access-date=2010-10-16}}</ref>

Most transmissions include a lockout mechanism to prevent reverse gear from being accidentally selected while the car is moving forwards. This can take the form of a collar underneath the gear knob which needs to be lifted or requiring extra force to push the gearshift lever into the plane of reverse gear.